Future mobile and wireless communication systems are demanding even faster data transmission and greater system capacity than present-day systems. To meet those demands, research is being conducted on techniques for transmitting and receiving data via multiple antennas. One of the transmission and reception techniques using the multiple antennas is a beamforming which focuses the beam of the transmit or receive antenna only upon a corresponding terminal.
FIG. 1 illustrates a beamforming data transmission of a conventional multi-antenna system.
In the multi-antenna system of FIG. 1, a base station transmits a preamble signal via multiple antennas and a terminal measures Channel Quality Indicator (CQI), such asquantized preamble Carrier-to-Interference-plus-Noise-Ratio (CINR) information, using the preamble signal and feeds the measured CQI back to the base station. The base station selects a Modulation and Coding Scheme (MCS) for a downlink using the feedback CQI information. Currently, equal weight is applied in sending the preamble signal.
With equal weight in the preamble signal transmission, a transmit beam power pattern 100 is spatially formed at right angles to the antenna array. Particularly, FIG. 1 shows the spatial beam power pattern of the waves including a specific frequency tone. When the channel is in Line-Of-Sight (LOS) range, user terminals receiving the strong preamble signal are subject to the spatial restriction. Even when the beam direction of the preamble does not face a user terminal 120, the user 120 of the strong channel strength is highly likely to report a small CQI to the base station. For example, while the second user 120 has a stronger channel strength than a first user 110 in terms of the distance, the beam direction of the preamble signal is consistent with the first user 110, and thus the second user 120 feeds a smaller CQI than the first user 110 to the base station.
Hence, it is highly likely to exclude the users 120 of the spatially inconsistent beam direction of the preamble signal, from the scheduling. Even when the user 120 is scheduled and assigned the higher MCS level, a lower MCS may be probably allocated to the user 120. Those problems become more serious for the lower-speed channel and greater number of the transmit antennas. Also, those problems degrade fairness of the data delivery and lower the data rate of the entire system.
To address those shortcomings, a Cyclic Delay Diversity (CDD) can be considered. CDD, which is one of signal transmission methods in a space-time domain, transmits data using multiple antennas by sending the signal over a first antenna without delay, sending the signal delayed by Δ via the second antenna, sending the signal delayed by 2Δ via the third antenna, and sending the signal delayed by (m−1)Δ via the m-th antenna. When the signal is delayed by (m−1)Δ, the phase of the signal changes by
      ⅇ          -      j        ⁢            2      ⁢      π      ⁢                          ⁢      k        N    ⁢      (          m      -      1        )    ⁢  Δin the k-th frequency tone. This signal phase change spatially alters the beam direction in the k-th frequency tone.
Since CDD forcibly generates the delays, the maximum delay of the multi-path increases far more. In the design of an Orthogonal Frequency Division Multiplexing (OFDM) system, a length of a Cyclic Prefix (CP) is usually greater than the maximum delay of the multi-path in order to avoid Inter-Symbol Interference (ISI). However, when CDD is adopted, the maximum delay interval of the multi-path is longer than the CP and thus causes ISI. As the number of the transmit antennas increases, the performance using CDD further deteriorates. The CQI acquired at the base station by receiving the preamble signal using CDD somewhat differs from the total strength of the actual channel of the terminal. As the frequency tone index increases, CDD slowly rotates the preamble beam in the space. Fast rotation increases the delay and causes ISI, and slow rotation disables the accurate measurement of the strength of the entire channel of the frequency tones.